Cells Part 2

Cytoskeleton Network of fibers extending throughout the cytoplasm. It organizes the cell’s structures and activities, anchoring many organelles. Helps to support the cell and maintain the shape. Interacts with motor proteins to produce motility. Inside the cell, vesicles can travel along tracks provided by the cytoskeleton It is composed of three types of molecular structures: Microtubules, Microfilaments, Intermediate Filaments Microtubules: the thickest of the three components of the cytoskeleton. Are hollow rods about 25 nm in diameter and about 200 nm to 25 microns long. Rolled into tubes called tubulin. They are stiff to provide rigidity and shape. They also provide an essential role in the equal separation of chromosomes to daughter cells in mitosis (centrioles). Also give organelles tracks for movement.

Centrosomes and Centrioles (Microtubules) In animal cells, microtubules grow out from a centrosome near the nucleus. In animal cells, the centrosome has a pair of centrioles, each with nine triplets of microtubules arranged in a ring. Found near the nucleus at right angles to each other. They produce spindles during cell division. Figure 6.22 Centrosome Microtubule Centrioles 0.25 μm Longitudinal section of one centriole Cross section of the other centriole Microtubules

Motor proteins “walk” vesicles along cytoskeletal fibers. ATP Receptor for motor protein Motor protein (ATP powered) Microtubule of cytoskeleton (a) (a) Motor proteins “walk” vesicles along cytoskeletal fibers. 0.25 μm Microtubule Vesicles Figure 6.21 Motor proteins and the cytoskeleton (b) SEM of a squid giant axon

Microfilaments (Actin Filaments) Microfilaments are solid rods about 7 nm in diameter, built as a twisted double chain of actin subunits. The structural role of microfilaments is to bear tension, resisting pulling forces within the cell. They form a 3-D network called the cortex just inside the plasma membrane to help support the cell’s shape. Bundles of microfilaments make up the core of microvilli of intestinal cells. Microfilaments that function in cellular motility contain the protein myosin in addition to actin. In muscle cells, thousands of actin filaments are arranged parallel to one another. Thicker filaments composed of myosin interdigitate with the thinner actin fibers.

Microfilaments Localized contraction brought about by actin and myosin also drives amoeboid movement. Cells crawl along a surface by extending pseudopodia (cellular extensions) and moving toward them. Cytoplasmic streaming is a circular flow of cytoplasm within cells. This streaming speeds distribution of materials within the cell. In plant cells, actin-myosin interactions drive cytoplasmic streaming.

Myosin motors in muscle cell contraction (c) Cytoplasmic streaming in Actin filament Myosin filament Myosin head Chloroplast 30 µm (a) Myosin motors in muscle cell contraction (c) Cytoplasmic streaming in plant cells Cortex (outer cytoplasm): gel with actin network 100 µm Figure 6.26 Microfilaments and motility Inner cytoplasm (more fluid) Extending pseudopodium (b) Amoeboid movement

Microfilaments (actin filaments) Microvillus 0.25 µm Plasma membrane Microfilaments (actin filaments) Figure 6.25 A structural role of microfilaments Intermediate filaments Figure 6.25

Intermediate Filaments Intermediate filaments range in diameter from 8–12 nanometers, larger than microfilaments but smaller than microtubules. They support cell shape and fix organelles in place. Intermediate filaments are more permanent cytoskeleton fixtures than the other two classes.

Table 6.1 The structure and function of the cytoskeleton

Cilia and Flagella Microtubules control the beating of flagella and cilia, microtubule-containing extensions that project from some cells. Cilia and flagella differ in their beating patterns. They grow out of basal bodies that anchor the cilia or flagellum. A motor protein called dynein, drives the bending movements. Length of whip like extension determines the name. Cilia are short, many. Used to move the organism or move materials past the cell. Flagella are long and usually only one or two are present in any individual cell.

Cross section of basal body Plasma membrane 0.1 μm Outer microtubule doublet Motor proteins (dyneins) Central microtubule Radial spoke Microtubules Cross-linking proteins between outer doublets (b) Cross section of motile cilium Plasma membrane 0.1 μm Basal body Triplet Figure 6.24 Structure of a flagellum or motile cilium 0.5 μm (a) Longitudinal section of motile cilium (c) Cross section of basal body

Direction of organism’s movement Motion of flagella Direction of swimming 5 μm (b) Motion of cilia Direction of organism’s movement Figure 6.23 A comparison of the beating of flagella and motile cilia Power stroke Recovery stroke 15 μm

Animation: Cilia and Flagella

Extracellular components: between cell coordination of cellular activities Most cells synthesize and secrete materials that are external to the plasma membrane. These extracellular structures are involved in a great many cellular functions. Includes Cell Wall in Plants, Extracellular Matrix in Animal Cells, Cell Junctions, and Plasmodesmata in Plant Cells.

Cell Walls of Plants The cell wall is an extracellular structure that distinguishes plant cells from animal cells Prokaryotes, fungi, and some unicellular eukaryotes also have cell walls The cell wall protects the plant cell, maintains its shape, and prevents excessive uptake of water Plant cell walls are made of cellulose fibers embedded in other polysaccharides and protein Plant cell walls may have multiple layers Primary cell wall: Relatively thin and flexible Middle lamella: Thin layer between primary walls of adjacent cells Secondary cell wall (in some cells): Added between the plasma membrane and the primary cell wall Plasmodesmata are channels between adjacent plant cells

Secondary cell wall Primary cell wall Middle lamella 1 μm Central vacuole Figure 6.27 Plant cell walls Cytosol Plasma membrane Plant cell walls Plasmodesmata

The Extracellular Matrix (ECM) of Animal Cells Animal cells lack cell walls but are covered by an elaborate extracellular matrix (ECM) The ECM is made up of glycoproteins such as collagen, proteoglycans, and fibronectin ECM proteins bind to receptor proteins in the plasma membrane called integrins The ECM has an influential role in the lives of cells. The ECM can regulate a cell’s behavior by communicating with a cell through integrins. The ECM around a cell can influence the activity of gene in the nucleus. Mechanical signaling may occur through cytoskeletal changes, that trigger chemical signals in the cell.

A proteoglycan EXTRACELLULAR FLUID complex Polysaccharide Collagen molecule Collagen Carbo- hydrates Fibronectin Core protein Plasma membrane Figure 6.28 Extracellular matrix (ECM) of an animal cell Proteoglycan molecule Microfilaments Integrins CYTOPLASM

Cell Junctions in Animals Neighboring cells in tissues, organs, or organ systems often adhere, interact, and communicate through direct physical contact Three types of cell junctions are common in epithelial tissues At tight junctions, membranes of neighboring cells are pressed together, preventing leakage of extracellular fluid Desmosomes (anchoring junctions) fasten cells together into strong sheets Gap junctions (communicating junctions) provide cytoplasmic channels between adjacent cells

Tight junctions prevent fluid from moving across a layer of cells. TEM 0.5 μm Tight junction Intermediate filaments Desmosome Figure 6.30 Exploring cell junctions in animal tissues Desmosome (TEM) 1 μm Gap junction Ions or small molecules Plasma membranes of adjacent cells Extracellular matrix TEM Space between cells 0.1 μm Gap junctions

Plasmodesmata in Plant Cells Plasmodesmata are channels that perforate plant cell walls. Through plasmodesmata, water and small solutes (and sometimes proteins and RNA) can pass from cell to cell. Cell walls Interior of cell Interior of cell 0.5 μm Plasmodesmata Plasma membranes Figure 6.29

Figure 6.UN03 Summary of key concepts: nucleus and ribosomes

Figure 6.UN04 Summary of key concepts: endomembrane system

Figure 6.UN05 Summary of key concepts: mitochondria, chloroplasts and peroxisomes